1. Field of the Invention
This invention relates to a feedback control system for the impedance control of an electric arc furnace, which system comprises at least one electrode, a hydraulic electrode actuator for adjusting the electrode, which actuator is adapted to be supplied with hydraulic fluid through a control valve, an impedance signal generator for delivering impedance signals to one input of the controller, a setpoint signal generator for delivering a setpoint signal representing a desired impedance to another input of the controller, and a correcting stage for correcting the controller gain of the controller.
2. Description of the Prior Art
In an electric arc furnace which is connected to a transformer the phase voltages and phase currents which are available on the secondary side of the transformer vary with the length of the electric arc or arc gap between a furnace electrode and the molten or meltable material in the furnace. As the ratio of the voltages and currents depends on the actual impedance, the impedance may be used as a controlled variable for a control of the length of the electric arc or arc gap by means of an electrode actuator for adjusting the electrode. For this purpose it is known from German Patent Specification 29 48 787 to calculate the actual impedance from the actual values of the measured phase voltages and phase currents and to compare the actual impedance with a predetermined desired impedance and in case of a deviation consisting of a difference between the desired and actual impedances to control the electrode actuator for the associated electrode by means of a corresponding manipulated variable delivered by a controller so that the deviation will be eliminated. In that case, electrical and mechanical oscillations may be excited in the electrode, which is held in a carrying arm, and the electrodes may be mechanically overloaded if such oscillations are excited at or near a resonant frequency of the oscillatable system consisting of the electrode and its carrying arm.
In an effort to ensure that oscillations of the electrodes and the associated carrying arms which are due to current fluctuations will be reduced to a permissible value, it has been proposed in Published German Application 28 41 857 to reduce the controller gain in dependence on the magnitude of any oscillations which may occur in the controlled variable, i.e., in the impedance which is calculated from the phase voltage and phase current, so that the substantially reduced controller gain will damp any oscillations of the controlled variable before they can affect the manipulated variable. But even that measure will not reliably prevent an occurrence of non-permissible electrode oscillations, particularly under different operating conditions.